IMO, sealing penatrations is what lap sealant is for. That's why new RV's use lap sealant, not tape. You want the ability to remove the sealant to replace the vent/cap, etc, if needed. The Eternabond is great for damage to the rubber membrane itself...a permanent repair that you will never need/want to remove.
Keep in mind that using tape around a round flange won't look so good. Again, use the tape for straight repairs. Use Dicor lap sealant for resealing. If only touching up surface cracks, you can clean with rubbing alcohol (and some elbow grease), and just go right over the top of the crack.
thebrakeman ('70), DW ('71), DD ('99), DD ('01), DD ('05)
2004 Surveyor SV261T (UltraLite Bunkhouse Hybrid)
2006 Mercury Mountaineer V8 AWD Premier
Equal-i-zer WDH (10k), Prodigy Brake Controller